1. Field of the Invention
The present invention relates generally to controlling light propagating in a wave guide. More particularly, the present invention relates to using gratings to cause mode coupling of light propagating in a wave guide.
2. Description of Related Art
Devices used in optical systems, such as in fiber optic communication systems and sensing systems, often benefit from the filtering or control of light propagating in a wave guide. Examples of such devices include, but are not limited to, source lasers, optical amplifiers, filters and other integrated-optical components. One method of controlling and filtering light utilizes diffraction gratings. Descriptions of such devices and how they benefit from diffraction gratings are described in T. Erdogan and V. Mizrahi, “Fiber Phase Gratings Reflect Advances in Lightwave Technology,” February 1994 edition of Laser Focus World.
There are three techniques typically used to create a diffraction grating in a wave guide to induce mode coupling or Bragg reflection. The most common method uses ultraviolet light to induce a refractive index change in an optical fiber. A system for producing a periodic refractive index change in the optical fiber is illustrated in FIG. 1. In FIG. 1 a first beam 104 of coherent ultraviolet “UV” light and a second beam 108 of coherent UV light are directed at a photosensitive optical fiber 112. At the intersection of the first beam 104 and the second beam 108, an interference pattern 116 is generated. The refractive index of the photosensitive optical fiber 112 changes with the intensity of the UV exposure, thus an index grating with a periodicity determined by the interference pattern 116 forms where the first coherent beam 104 and the second coherent beam 108 intersect.
A second technique for creating a grating in an optical fiber involves etching a periodic pattern directly onto an optical fiber. In one embodiment, a photomask is used to generate a periodic pattern in a photolithographic process. An acid etch etches the grating or periodic pattern into the optical fiber. Such photomasks and etching are commonly used in semiconductor processes.
A third technique to control light in a waveguide is used in semiconductor waveguides. In one embodiment, a layered growth is formed on the semiconductor wave guide to generate light reflection in the wave guide.
The described techniques for creating a grating on or in a wave guide are permanent. The gratings have a fixed periodicity at a fixed location on the waveguide that cannot be easily changed. Thus, a particular wave guide and grating combination will have a predetermined transmission characteristic. In order to change the characteristic, the entire wave guide segment containing the grating is typically replaced with a wave guide segment having a different transmission characteristic. Replacing wave guide segments is a cumbersome process requiring that each end be properly coupled to the light source and the light receiving device.
Thus, an improved system and method to control light propagating in a wave guide is needed.